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PERSPECTIVES

A Reminder of the Limitations in Using Beer's Law to Estimate Daily Radiation Interception by Vegetation

Agronomy Physiology Lab., Univ. of Florida, P.O. Box 110965, Gainesville, FL, 32611-0965

^* Corresponding author (trsincl{at}ifas.ufl.edu)

Radiation extinction coefficients for leaf canopies are often calculated using Beer's Law based on midday measurements of radiation interception. However, the assumption of Beer's Law is not appropriate for leaf canopies and this empirical approach needs to be used with caution. Alternatively, classic derivations of radiation interception by leaf canopies have resulted in a similar exponential form defined as a function of time of day, day of year, and latitude. A common experimental approach of determining the extinction coefficient from midday measurements results in a minimum coefficient that underpredicts the total daily radiation interception. Two approaches are explored to improve the estimates of daily radiation interception under a clear sky. The first approach compared extinction coefficients calculated for midday against ones calculated as representative of radiation interception over the entire day. A linear correlation was found between the two extinction coefficients so that the midday extinction coefficient could be corrected to obtain a coefficient representative of the entire day under a clear sky. The second approach estimated the time of day when measurements could be made to obtain directly an extinction coefficient representative of the entire day. While such times in the day could be identified, this approach is impractical because the periods for taking these measurements are short and dependent on leaf angle.

Abbreviations: G, leaf shadow projection • k_i, instantaneous extinction coefficient • k_d, daily extinction coefficient • L, leaf area index • PAR, photosynthetically active radiation • R_b, irradiance transmitted through the leaf canopy • R_b^o, irradiance incident on the leaf canopy • , leaf angle • ß, solar angle